Many image coders support Interframe coding in which movement between the images of a sequence is estimated in order for the most recent image to be coded relative to one or more preceding images.
Each image of the sequence can also be coded without reference to the others. This is known as Intraframe coding and exploits spatial correlations in an image. For a given transmission bit rate from the coder to the decoder, it achieves lower video quality than Interframe coding because it does not make use of temporal correlation between images of the sequence.
A sequence commonly has its first image Intraframe-coded and subsequent images Interframe-coded. Information included in the output stream from the coder indicates the Intraframe-coded and Interframe-coded images and, when Interframe-coded, which reference image(s) to use.
A number of existing coding methods code a current image portion by determining representative information known as descriptors that consist of information relating to the pixels, for example, such as the luminance and the chrominance, or movement vectors for coding mode-choice information.
Some of those descriptors, in particular the movement vectors, can be predicted. It is then possible to analyze image portions to obtain predicted descriptors that are thereafter compared with current descriptors to extract a residue representing the difference between the predicted and current descriptors. Only this residue needs to be transmitted to a decoder.
The corresponding decoding methods are adapted to determine the predicted descriptors, such as the predicted movement vectors, locally and combine them with the residue received from the coder to obtain the current descriptors and therefore the current image portion.
Thus in such coding the stream between the coder and the decoder contains only the residue, and where applicable the reference of the image portions to use.
However, the prediction function that is used is sometimes not the optimum function. Employing groups of prediction functions that can be used in the coder and the decoder can overcome this problem. Each of the functions is tested in the coder before the coder selects one of them, generally the function producing the minimum residue.
In particular, among the descriptors, the movement vectors require a high bandwidth, in particular because of their accuracy, and are thus liable to be transmitted using a residue.
It is therefore necessary to include in the coder output stream an identifier of the prediction function used to enable the decoder to apply the correct prediction function.
The bandwidth allocated to the identifier of the prediction function is not negligible and increases with the size of the group from which the function is obtained.
This problem is addressed in IEEE Transactions on Image Processing, Vol. 8, no. 8, August 1999, by Sung Deuk Kim and Jong Beom Ra, who propose a particular coding system for the identifier of the prediction function used for the movement vectors.
Thus an increase in the size of the group of usable prediction functions improves prediction quality, but requires the allocation of a greater bandwidth for the identifier.